Integrated primaryKey extension to NSEntityDescription and refactored cache strategy

classes to eliminate issues with duplicated objects. closes #611, #612, #613, #618

* NSEntityDescription is now aware of the primaryKeyAttribute. Can be configured via
Interface Builder within Xcode or programatically.
* Added findByPrimaryKey: interface to the Core Data extensions.
* Relaxed dependencies on RKManagedObjectMapping across the system now that primaryKey is
available without a reference to the mapping.

Code/CoreData/RKInMemoryManagedObjectCache.m

@@ -0,0 +1,37 @@
+//
+//  RKInMemoryManagedObjectCache.m
+//  RestKit
+//
+//  Created by Jeff Arena on 1/24/12.
+//  Copyright (c) 2012 RestKit. All rights reserved.
+//
+
+#import "RKInMemoryManagedObjectCache.h"
+#import "RKLog.h"
+
+// Set Logging Component
+#undef RKLogComponent
+#define RKLogComponent lcl_cRestKitCoreData
+
+static NSString * const RKInMemoryObjectManagedObjectCacheThreadDictionaryKey = @"RKInMemoryObjectManagedObjectCacheThreadDictionaryKey";
+
+@implementation RKInMemoryManagedObjectCache
+
+- (NSManagedObject *)findInstanceOfEntity:(NSEntityDescription *)entity
+                  withPrimaryKeyAttribute:(NSString *)primaryKeyAttribute
+                                    value:(id)primaryKeyValue
+                   inManagedObjectContext:(NSManagedObjectContext *)managedObjectContext {
+    NSAssert(entity, @"Cannot find existing managed object without a target class");
+    NSAssert(primaryKeyAttribute, @"Cannot find existing managed object instance without mapping");
+    NSAssert(primaryKeyValue, @"Cannot find existing managed object by primary key without a value");
+    NSAssert(managedObjectContext, @"Cannot find existing managed object with a context");
+    RKInMemoryEntityCache *cache = [[[NSThread currentThread] threadDictionary] objectForKey:RKInMemoryObjectManagedObjectCacheThreadDictionaryKey];
+    if (! cache) {
+        cache = [[RKInMemoryEntityCache alloc] initWithManagedObjectContext:managedObjectContext];
+        [[[NSThread currentThread] threadDictionary] setObject:cache forKey:RKInMemoryObjectManagedObjectCacheThreadDictionaryKey];
+        [cache release];
+    }
+    return [cache cachedObjectForEntity:entity withAttribute:primaryKeyAttribute value:primaryKeyValue inContext:managedObjectContext];
+}
+
+@end